In a client-server system, clients make requests which are responded to by a server. In this relationship, the server is passive. In other words, if a client desires data from a database, it makes a request to the server, which only then furnishes the information to the client. After making the requests, the client waits for responses by the server. The server does not initiate interaction with the client on its own. This type of system is commonly referred to as a "pull" data system. In such a system, once a client comes on-line, the client must request information before any is provided to it. There is no automatic sending of information to a client once it has been detected as being on-line. Situations exist where it would be desirable to have information "pushed" out to a client, such as when the client is operating at a remote location and comes on-line. For instance, changes in service orders or plans may occur, such that notification of the client is necessary.
Currently, client-server systems have rarely been expanded for use outside individual business. This limited expansion is due to the dependence of client-server applications on the reliable, high-speed networks found within the business entity (e.g., corporation). As a result, information tools for mobile personnel rarely connect remote users with information that may be critical to their work.
Today, mobile agents outside the boundaries of high-speed corporate networks have communication channels available that typically include wireless networks or phone lines while in the field and local area networks (LANs) while in the office. Both wireless networks and phone lines are orders of magnitude more constrained than traditional high-speed LANs. Table 1 illustrates the dramatic discrepancy between bandwidth (kilobytes per second) and an information transfer between a client and a server on various media, referred to herein as a round-trip.
TABLE 1 ______________________________________ Networks Bandwidth Latency ______________________________________ LAN 5,000-10,000 kbps .0005-.001 seconds Circuit Switched Land- 2.4K-28.8 kbps .2-.5 seconds Line Modem Wireless WAN 2K-9 kbps 4-10 seconds ______________________________________
As shown in Table 1 above, a round-trip of information between the client and the server takes only 0.0005 seconds on a LAN and only 0.5 seconds on a dial-up network. However, on a wireless network, such a round-trip of information could range from 4 to 10 seconds.
Most client-server applications perform numerous (tens) of round-trips in a single application unit of work. Although this may be easy on a LAN, the many round-trips required for most applications cause a delay to be introduced by a wireless network that is no longer the 4-10 seconds required for a single data round-trip. It is actually 10-50 times longer for each given application unit of work completed. These delays are intolerable to a user. It is desirable to reduce the time necessary to complete round-trips when performing data or information transfers with a wireless network.
Compounding the difficulty of utilizing mobile-based communications in client-server systems, mobile networks disconnect or fade in and out of coverage regularly, neither of which is tolerated by the connection-oriented nature of client-server applications. The client-server system relies on stable connections between the client and the server. If the client and server become disconnected, the client must engage in another log-in sequence and then re-aquire the work that was in progress when the connection disappeared. It is desirable to avoid performing log-in sequences every time a client-server connection becomes disconnected.
In summary, today's client-server applications do not work well when applied to mobile communications.